My understanding is that while the upfront investment is bigger, a tramway is cheaper to operate than a bus route with the same capacity. Also, when a bus route is upgraded to a tram line, the passenger counts and thus ticket revenues increase. Further, the value of the land around the tram line will increase, and the city can offset some or all of the investment with revenues from selling, renting or re-zoning land/buildings.

Similarly, a tram costs more than a bus upfront, but a tram will last much longer and the drivers' salary costs will be divided among a bigger number of passengers. [Also, trams have no fuel or battery costs!]

Regarding Turku specifically: https://ratikkafaktat.fi/fi/fakta/95/